1. Field of the Invention
The invention relates to a method for preparing nano-composite materials, and more particularly to a method for preparing an insulating varnish.
2. Description of the Related Art
Typical methods for preparing thermal conductive and insulating polymer material include blending methods and nano-compositing methods.
The blending methods include: powder blending, solution blending, double roll blending, and melt blending.
1) Powder blending is mixing the polymer powder, such as HDPE and PS, with thermal conductive filler powder in a certain mass ratio and shaping by die-forming or melt-casting.
2) Solution blending is dissolving the polymer in a solvent to form a solution, evenly dispersing a conductive filler in the solution, evaporating the solvent, and shaping by melt-casting or die-forming.
3) Double roll blending is mixing the polymer and the filler at a certain temperature in a roll mill and shaping by a hot pressing machine.
4) Melt blending is mixing the conductive filler powder and melted polymer by using an injection equipment and shaping.
The nano-compositing methods include direct blending-nano-compositing method and intercalation compositing method.
1) The direct blending-nano-compositing method is mixing the conductive nano powder with the polymer by using a machine for nano-compositing.
2) Intercalation compositing method is providing laminated and highly productive inorganic filler, such as graphite and boron nitride, blending polymer in a melted state or solution state with the laminated filler, dispersing the laminated filler of a nano size in the polymer under the force of the mechanical chemistry and thermodynamics to realize a nano-compositing, maintain a local ordered arrangement of the inorganic filler, and improve the conductive property.
Conventional insulating varnish is prepared by using an organic polymer as a base material, and adding proper amount of color agent, lubricate, diluent, and flatting agent for compositing. Highly thermal conductive and insulating materials, such as highly thermal conductive and insulating plastic and silicon rubber, added with thermal conductive inorganic particles have been wide applied, but insulating varnish has not yet applied in practical use. The above methods for preparing composite insulating materials added with highly conductive inorganic particles have disadvantages as follows: powder blending is not suitable for polymer not in solid particles; composite materials prepared by the double roll blending method and the melt blending method have low thermal conductivity; because nano-particles have large surface area and high surface activity, they easily agglomerates and are difficult to be evenly dispersed in the melted polymer by using the nano-composite method; the intercalation composite method is particularly suitable for the laminated and highly conductive inorganic filler rather than particles of other shapes, thereby restricting the application of the method; the solution blending method and the direct blending-nano-compositing method employ a high speed mixing device or ultrasonic dispersion device to evenly dispersing the conductive nano-filler, composite effects thereof are restricted by the particle size and surface activity of the nano-particles, physical interactions between particles, intermolecular force between nano-particles and polymer, particle size of polymer, and composite technology. The insulating material is required to have excellent heat resistance, but the conventional insulating varnish has poor thermal conductivity.
Therefore, it is important to provide a method for preparing highly thermal conductive and insulating varnish that is simple and economic, and has significant practical effect.